The present invention relates generally to a hybrid rocket propulsion system. More particularly, the invention discloses a hybrid propulsion system that provides a gasified oxidizer to a solid propellant fuel in a controlled fashion to safely and efficiently regulate burning, thereby controlling thrust and performance of the rocket.
A hybrid propulsion system combines both solid propellants and liquid propellants into a single propulsion system. Traditional liquid propulsion systems incorporate all liquid propellants while solid propulsion systems include only propellants in solid forms. A third class of propellant systems may be referred to as hybrid systems which utilize one propellant in the form of a solid propellant grain encased within a pressure housing while the opposite propellant is delivered into the pressure housing in either a liquid or gaseous form. Combustion is initiated upon contact between the two propellants in the presence of an ignition source. The resulting high-pressure products of combustion may be used to perform work. The propulsion system discussed in this application will be described in the environment of a rocket engine. In such systems the combustion products are expanded through a convergent divergent (laval) nozzle, and accelerated to a high velocity thereby providing a reaction force that may act as the rocket thrust. Generically either the fuel or the oxidizer may be used as the solid propellant while the opposite propellant is delivered in fluid form.
The hybrid rocket is similar in some respects to solid fuel ramjets and to ducted rockets. Solid fuel ramjets incorporate a solid fuel grain contained within a motor casing. As the ramjet travels through the atmosphere, its forward velocity delivers ram air to the fuel grain in a forced manner. Oxygen within the air serves as the oxidant to achieve combustion.
Ducted rockets function similarly to solid fuel ramjets except that some oxidizers are contained within the motor casing and may be used for such purposes as ignition of the fuel, stabilization of the flame, and/or to augment burning by providing additional oxidants to support combustion. The major difference between hybrid rockets and ducted propulsion devices is that the latter receive some, if not all, of its oxidizers from the medium through which it flies. In contrast, the hybrid rocket, like other true rockets, is completely self-contained carrying all of its own propellants and therefore is not dependent upon the surrounding medium to support combustion.
Hybrid rockets have certain advantages over both liquid and solid rockets. In contrast to bipropellant liquid rockets, the hybrid requires only a single propellant supply system. This greatly simplifies the mechanics of the systems since only a single propellant tank is required along with a single propellant turbopump, a single set of connecting lines and valves, as well as a single pressurization systems, etc. Such benefits are further amplified if there is a need for redundant propellant delivery systems. Therefore, hybrids represent a major simplification of the vehicle and pay substantial dividends in the form of better safety, reliability, and economy. Further, since one of the propellants (usually the fuel) is in the form of a solid, the necessary size of the propellant storage system is considerably reduced. Furthermore, safety is considerably enhanced since it is extremely unlikely that propellant mixing will occur in areas that are likely to produce explosions and/or fires such as those known to occur in bipropellant liquid rockets.
One principal advantage that the hybrid has over a solid motor is that the thrust of the hybrid can be easily controlled, adjusted, or terminated. For example, the rate of injection of the liquid propellant can be controlled which facilitates throttling over a wide thrust range. Further, hybrid rocket motors avoid the mixing and curing problems encountered when combining fuel and oxidizer constituents into a single propellant grain.
Another major advantage of the hybrid over purely solid propellants is that solid oxidizers inherently consist of a high percentage of inert matter, which necessarily results in lower propellant performance. Liquid oxidizers, on the other hand, are virtually all oxidizer and therefore have lower weight requirements and higher burning temperatures. Consequently, a well designed hybrid propulsion system using a liquid oxidizer in combination with a solid fuel grain can ideally approach the performance of a liquid rocket and out perform conventional solid fuel rocket motors.
Contemporary hybrid rockets utilize liquid oxygen sprayed under pressure into the combustion region of a solid fuel grain. While such an approach achieves the advantages discussed above, it also has several drawbacks which have limited the practicality of hybrid systems to date. For example, injection of cold liquid oxygen into the region of the fuel grains frequently result in the liquid droplets eroding the grain in an undesirable pattern. Specifically, such erosion makes it difficult to control the location and rate of burning, as well as making it difficult to control the mixture ratio. Thus, such systems typically have less than optimum performance. Additionally, such systems incur nozzle streaking and/or erosion problems which stem from the presence of unburned oxygen.
Therefore, there is a need for a hybrid rocket system which is amenable to accurate combustion control and capable of overcoming the aforementioned difficulties with current hybrids.